How can I reassign or move vertexes of a vertex group
tuples are read only you cant reasign its value. 
try to use to_list() instead... i think.
[list(vert) for vert in  vertstarg]
then seems like you are doing nothing to your verts but selecting 

grab the header bar down (the file menu) to see some info from operators. or go to de scripting window, there is on the top also.
almost every thing you do inside blender is shown on that "console"
see https://www.youtube.com/watch?v=2j75AM1Mttc